Hydrogen bis(fluorosulfonyl)imide (HFSI) may be used in synthesizing any of a variety of compounds, including alkali-metal bis(fluorosulfonyl)imide (FSI) salts, such as lithium FSI (LiFSI), sodium FSI (NaFSI), and potassium FSI (KFSI), among others. Typically, synthesis of alkali-metal FSI salts starts with obtaining or synthesizing a crude HFSI. Examples of methods of synthesizing HFSI include reacting ClSO2NHSO2Cl with a fluorinating agent, such as AsF3, SbF3, BiF3, or anhydrous HF, under conditions sufficient to synthesize the HFSI or reacting fluorosulfonic acid with urea under conditions sufficient to synthesize the HFSI. However, the HFSI so synthesized may be considered “crude HFSI”, because the HFSI in the reaction product is contaminated with various impurities, such as fluorosulfonic acid (FSO3H), hydrochloric acid (HCl), hydrogen fluoride (HF), and sulfuric acid (H2SO4), that are byproducts of the corresponding reaction.
When crude HFSI is used to synthesize alkali-metal FSI salts, the impurities from the synthesis of HFSI result in impurities with the alkali-metal FSI salts. Using LiFSI as an example, when LiFSI is synthesized by neutralizing crude HFSI, impurities such as the HF, FSO3H, HCl, and H2SO4 are converted into the corresponding lithium salt to produce LiF, FSO3Li, LiCl, and Li2SO4. Depending on the desired use of an alkali-metal FSI salt synthesized from crude HFSI, the impurities resulting from the impurities in the crude HFSI can be detrimental to that use.
Again using LiFSI as an example, LiFSI salt may be used in electrolytes for lithium-ion batteries due to it having desirable properties for that use. Examples of these desirable properties include, but are not limited to, far superior stability compared to LiPF6 (a common electrolyte salt for lithium-ion batteries), excellent solubility, ionic conductivity similar to LiPF6, cost-effectiveness, environmental benignity, and ability to form a favorable solid electrolyte interface (SEI). However, the impurities, for example, LiF, FSO3Li, LiCl, and Li2SO4, in the LiFSI salt products resulting from impurities, for example, HF, FSO3H, HCl, and H2SO4, in the underlying crude HFSI can have undesirable effects on the operation and life of lithium-ion batteries if their levels are not below certain upper limits. Consequently, it would be desirable to be able to reliably remove impurities, such as the synthesis impurities noted above, from crude HFSI prior to using the HFSI to synthesize LiFSI, another alkali-metal FSI, or other composition.